Issue 39 October 2021 - Journal - Page 107
If these are sufficiently large and in the tensile stress
zone (towards the centre of the glass thickness), they
can disrupt the stresses built into toughened glass to
an extent that the glass fractures from around the
inclusion. Often this will occur during manufacture,
but it can also occur sometime after manufacture, usually fairly quickly, but it may be a matter of months.
Toughened glass containing a non-critical inclusion
which survives longer than a few months is very
unlikely to fracture from the inclusion in service.
• Thermally toughened soda lime silicate safety glass
– EN 12150
• Heat soaked thermally toughened soda lime silicate
safety glass – EN 14179
The presence of an inclusion within annealed glass is
not a problem. It is only of concern when in the
tensile stress zone of a thermally treated product.
This article covers the spontaneous breakage of
thermally treated glasses, together with types of inclusions, the rate of occurrence and associated risks.
Types of Inclusions- Critical
Background nickel sulphide has two main states, one
of which is stable at high temperatures and one which
is stable at lower temperatures. When glass is thermally treated the nickel sulphide transforms to the
high temperature state during the heating process,
but the glass is cooled rapidly which does not allow the
reverse transformation to the low temperature state.
This reverse transformation occurs over a period of
time, accompanied by an increase in volume.
For the purpose of this article the following definitions
• Spontaneous breakage (also referred to as spontaneous fracture) - An apparent unexplained fracture
that can occur in heat treated glasses without an
obvious external influence.
• Inclusions - An inclusion that by virtue of its size and
position in the thermally treated glass can cause
• NOTE: These can be of various materials that are either critical (e.g. nickel sulphide) or non-critical (e.g.
refractory stone, un-melted frit)
• Critical inclusions - An inclusion or small impurity in
the glass that can undergo a phase change which may
lead to fracture of thermally toughened soda lime
silicate glass sometime after toughening.
• The nickel sulphide inclusion is large enough, and
• Within the tensile (central) portion of the thermally
treated glass, it can cause fracture at some time after
All types of thermally treated glass, i.e. heat strengthened, thermally toughened, can be subject to spontaneous breakage as the result of the presence of critical
nickel sulphide inclusions. However, the risk of spontaneous breakage due to the presence of a critical
nickel sulphide inclusion can be significantly reduced
by using heat soaked thermally toughened soda lime
silicate safety glass in accordance with EN 14179.
• NOTE: Failure is also possible in heat strengthened
soda lime silicate glass.
• Nickel Sulphide inclusions -The most common type
of critical inclusion found within thermally treated
soda lime silicate glass.
• Level of associated risk - Risk of spontaneous
breakage of thermally treated soda lime silicate glass
on a statistical basis due to the presence of critical
Rate of Occurrence of Fracture Due to Critical
There are no definitive and proven concentration
levels for critical inclusions in any manufacturers thermally toughened soda lime silicate safety glass.
Types of Inclusions- Non – Critical
These can be one of the following:
• Un-melted frit
• Fragments of refractory block
• Inclusion that does not undergo phase change
NOTE: Glass manufacturers have taken action to reduce
nickel contamination of the annealed glass since NiS was
shown to be a cause of spontaneous fracture of toughened
Consequently, the incidence of NiS in glass has now
Spontaneous breakage of thermally toughened soda
lime silicate safety glass due to critical inclusions remains statistically unlikely for the large quantities of
glass supplied and installed in buildings.
Reducing Spontaneous Breakage due to Critical
Heat soaked thermally toughened soda lime silicate
safety glass is manufactured by taking thermally
toughened panes and subjecting them to the heat soak
process cycle. The heat soak process cycle consists of a
heating phase, a holding phase and a cooling phase.
This process encourages unstable α phase to convert
to the β stable phase and force the glass to fracture
within the heat soaking oven. The heat soak process
Above, example of un-melted frit
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